A gyroscope is a sensor that measures the rate of rotation of an object. The concept of a vibrating MEMS (Micro-Electro-Mechanical System) gyroscope is to generate the momentum of a proof-mass to induce and detect the Coriolis force. A Coriolis force is applied to the proof-mass in motion when an angular rate is applied. The Coriolis force Fc is the product of the proof-mass ‘m’, the input rate ‘Ω’ and the mass velocity ‘v’. The direction of the Coriolis force is perpendicular to the proof-mass.
The basic architecture of a vibratory gyroscope is comprised of a drive-mode oscillator that generates and maintains a constant linear momentum of the proof-mass, and a sense mode circuit that measures the sinusoidal Coriolis force induced due to the combination of the drive oscillation and any angular rate input. The majority of rate gyroscopes utilize a vibratory proof-mass suspended by springs above a substrate. The objective being to form a vibratory drive oscillator coupled to an orthogonal sense system detecting the Coriolis force.
Since the Coriolis Effect is based on conservation of momentum, the drive-mode oscillator circuit is implemented to provoke the oscillation of the proof-mass which is the source of this momentum.
FIG. 1 illustrates a simplified block diagram of an example of such a drive-mode oscillator circuit 100. The drive-mode oscillator circuit 100 in the illustrated example comprises a capacitance to voltage (C2V) circuit 110 arranged to convert a capacitance change of a MEMS drive measurement unit (DMU) (not shown) caused by the displacement of the proof-mass to a voltage measurement signal. An integrator 120 receives the voltage measurement signal and phase shifts it by, for example, 90° to compensate for the phase lag of the MEMS. A voltage gain amplifier (VGA) 130 receives the phase shifted voltage signal and outputs an actuation voltage signal to a drive actuation unit (DAU) (not shown) of the MEMS. An automatic gain control (AGC) circuit 140 provides a control signal to the VGA 130 to control the amplitude of the actuation voltage signal output thereby.
When a MEMS gyroscope is initially turned on, it takes a significant amount of time to achieve an oscillating displacement range for the proof-mass required for obtaining meaningful measurements. In order to avoid delays in the use of such MEMS gyroscopes, it is known to implement a ‘standby mode’ in which oscillation of the proof-mass is maintained by the drive-mode oscillator circuit, whilst the sensing circuit(s) is/are powered down to conserve power. Such a standby mode is also known as ‘drive-running mode’, ‘ready-mode’, ‘sleep-mode’, etc.
However, maintaining such a drive-mode oscillator circuit powered up and driving the proof-mass still consumes a significant amount of power, which in many electronic applications is undesirable.